Directional valves of this type are used in internal combustion engines, for example for actuating hydraulic camshaft adjusters or switchable cam followers, for example switchable drag levers, bucket tappets or roller tappets. The directional valves comprise an actuating unit and a valve section. The actuating unit can be, for example, an electromagnetic or hydraulic actuating unit. The valve section represents the hydraulic section of the directional valve, a feed connection, at least one working connection and a tank connection usually being configured on said section. Defined connections of the valve section can be hydraulically connected to one another in a targeted manner by means of the actuating unit and therefore the pressure medium flows can be directed.
For the use of a directional valve for controlling a camshaft adjuster, said directional valve is configured in the normal case as a 4/3-way proportional valve. A proportional valve of this type is disclosed, for example, in DE 100 27 080 A1.
The valve section comprises a valve housing and a control piston which is arranged such that it can be displaced axially in said valve housing. The valve housing is usually arranged within a cylindrical, blind bore-like receptacle of the cylinder head or a central hole of the camshaft adjuster. Four annular grooves are formed on the outer circumferential face of the valve housing, in the groove bottoms of which annular grooves openings are formed which serve as pressure medium connections. One pressure medium channel which opens into the respective annular groove is formed per annular groove in the cylinder head. Pressure medium can pass into the interior of the valve housing or can escape from the interior of the valve housing via the connections. A control piston is arranged axially displaceably in the interior of the valve housing, the external diameter of the control piston being adapted to the internal diameter of the valve housing. Furthermore, annular grooves are likewise formed on the control piston, via which annular grooves adjacent pressure medium connections can be connected to one another.
A coil and an armature are arranged in the interior of the actuating unit. The armature is displaced in the axial direction by applying current to the coil, this movement being transmitted to the control piston by means of a push rod which is fastened to the armature. Furthermore, a spring acts on the control piston, which spring is supported on the valve housing and loads the piston with a force in the axial direction toward the actuating unit.
Directional valves for actuating switchable cam followers are usually configured as control valves. A control valve of this type is known, for example, from DE 103 59 363 A1, in an embodiment as a 3/2-way control valve. The function and the configuration of the electromagnetic actuating unit are largely analogous to those of the proportional valve.
In this case, a feed connection, a working connection and a tank connection are configured on the valve section. The working connection communicates both with the feed connection and with the tank connection via in each case one opening which is configured as a valve seat. Furthermore, a control piston is arranged within the valve housing, on which control piston two closing elements are formed. Depending on the position of the control piston within the valve housing, each closing element can shut off or release the pressure medium flow through one of the valve seats. Depending on the axial position of the control piston, the working connection can thus be connected selectively to the feed connection or to the tank connection. Here, the axial position of the control piston is in turn fixed via the axial position of the armature relative to the second magnet yoke.
In DE 100 27 080 A1, each of the hydraulic connections is assigned an annular filter. Each annular filter is positioned in one of the annular grooves, each annular filter extending along the entire circumferential direction of the circular groove bottom. The annular filter has a frame element made from an elastic plastic. The frame element has two part elements which extend in the circumferential direction of the valve housing, lie opposite one another in the axial direction and are connected to one another via transverse struts. A filter fabric is arranged between the part elements and the transverse struts. For example, the filter fabric can be inserted into the injection mold as a molded part during the production of the frame element and can be encapsulated with the plastic by injection molding. The frame element is of elastic configuration and, in the circumferential direction, has two open ends which lie opposite one another and can be connected to one another by means of a closure after the insertion of the annular filter into the groove.
It is disadvantage of this embodiment that in the case of a slight faulty orientation of the annular filter with respect to the valve housing, the transverse struts of the frame element can be arranged radially with respect to the openings of the connections or directly adjacently to the opening of the pressure medium channel which is formed in the cylinder head and with which the annular groove communicates. The faulty orientation can be caused during operation, for example, by the flow forces of the pressure medium and the play between the annular groove and the frame element. This results in considerable throttling of the pressure medium flow through the directional valve, which leads to an undesired, temporally variable change in the directional valve characteristics, and therefore in the worst case to the failure of the controller of the connected hydraulic consumer.